Add doc aliases for "delete"

This patch adds doc aliases for "delete". The added aliases are
supposed to reference usages `delete` in other programming
languages.

- `HashMap::remove`, `BTreeMap::remove` -> `Map#delete` and `delete`
  keyword in JavaScript.

- `HashSet::remove`, `BTreeSet::remove` -> `Set#delete` in JavaScript.

- `mem::drop` -> `delete` keyword in C++.

- `fs::remove_file`, `fs::remove_dir`, `fs::remove_dir_all`
  -> `File#delete` in Java, `File#delete` and `Dir#delete` in Ruby.

Before this change, searching for "delete" in documentation
returned no results.

library/alloc/src/collections/btree/map.rs

@@ -823,6 +823,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     /// assert_eq!(map.remove(&1), Some("a"));
     /// assert_eq!(map.remove(&1), None);
     /// ```
+    #[doc(alias = "delete")]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn remove<Q: ?Sized>(&mut self, key: &Q) -> Option<V>
     where

library/alloc/src/collections/btree/set.rs

@@ -770,6 +770,7 @@ impl<T: Ord> BTreeSet<T> {
     /// assert_eq!(set.remove(&2), true);
     /// assert_eq!(set.remove(&2), false);
     /// ```
+    #[doc(alias = "delete")]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool
     where